[python] Refine pyenv based virtualenv management

Now that pyenv plugins availability is detected by directly probing
'pyenv', we need to rely on the same mechanism consistently.

Further, we perform available pyenv plugin scan with native zsh
techniques instead of relying on external commands.

modules/python/init.zsh

@@ -92,18 +92,25 @@ if (( $+VIRTUALENVWRAPPER_VIRTUALENV || $+commands[virtualenv] )) && \
   # Disable the virtualenv prompt.
   VIRTUAL_ENV_DISABLE_PROMPT=1
 
+  # Create a sorted array of available virtualenv related 'pyenv' commands to
+  # look for plugins of interest. Scanning shell '$path' isn't enough as they
+  # can exist in 'pyenv' synthesized paths (e.g., '~/.pyenv/plugins') instead.
+  local -a pyenv_plugins
+  if (( $+commands[pyenv] )); then
+    pyenv_plugins=(${(@oM)${(f)"$(pyenv commands --no-sh 2>/dev/null)"}:#virtualenv*})
+  fi
+
+  if (( $pyenv_plugins[(i)virtualenv-init] <= $#pyenv_plugins )); then
     # Enable 'virtualenv' with 'pyenv'.
-  if (( $+commands[pyenv] )) && \
-     pyenv commands | command grep -q virtualenv-init
-  then
     eval "$(pyenv virtualenv-init -)"
-    # Optionall activate 'virtualenvwrapper' with 'pyenv' is available.
-    if (( $#commands[(i)pyenv-virtualenvwrapper(_lazy|)] )); then
-      pyenv "${${(@O)commands[(I)pyenv-virtualenvwrapper(_lazy|)]}[1]#pyenv-}"
+
+    # Optionally activate 'virtualenvwrapper' plugin when available.
+    if (( $pyenv_plugins[(i)virtualenvwrapper(_lazy|)] <= $#pyenv_plugins )); then
+      pyenv "$pyenv_plugins[(R)virtualenvwrapper(_lazy|)]"
     fi
   else
-    # Fallback to 'virtualenvwrapper' without 'pyenv' wrapper in '$path'
-    # and other known locations on a Debian based system.
+    # Fallback to 'virtualenvwrapper' without 'pyenv' wrapper if available
+    # in '$path' or in an alternative location on a Debian based system.
     virtenv_sources=(
       ${(@Ov)commands[(I)virtualenvwrapper(_lazy|).sh]}
       /usr/share/virtualenvwrapper/virtualenvwrapper(_lazy|).sh(OnN)
@@ -114,6 +121,8 @@ if (( $+VIRTUALENVWRAPPER_VIRTUALENV || $+commands[virtualenv] )) && \
 
     unset virtenv_sources
   fi
+
+  unset pyenv_plugins
 fi
 
 # Load PIP completion.